The present invention relates to a processing apparatus for radiographic image signals, and particularly to image processing (frequency processing) in a medical radiographing system.
A radiographic image such as an X-ray image is widely used for medical purposes. There is a known radiographic method in which a fluorescent material layer (fluorescent screen) is irradiated with radioactive rays passed through an object, and a film to be developed, on which a silver halide photosensitive material is coated, is irradiated with a visible light emitted from the fluorescent screen.
Recently, due to the progress of radiographic image diagnosis technology, a method has been invented by which the aforementioned radiograph is scanned, a radiographic image information recorded on the radiograph is read out, and the read out image information is regenerated on a CRT or a photosensitive material after it has been digitalized. Due to the aforementioned, various diagnostic information can be obtained from radiography at one time, and thereby diagnosis can be improved and an exposure dose can be lowered. This is expected from the following, in which the radiographic image information can be stored, and information retrieval can be conducted efficiently.
In the radiographic image information reading apparatus using a film, a photographing film in which a radiographic image is recorded, is exposure-scanned by a reading light, and the reflected light or the transmitted light at the time is detected by an photo-detector and is converted into an electric signal.
On the other hand, a method has been invented in which the radiographic image information is obtained without using a radiographic film made of a silver halide photosensitive material. In this method, radioactive rays which have passed through a subject are made to be absorbed into some kind of a fluorescent substance, then the fluorescent substance is excited by, for example, light or heat energy, thereby radioactive ray energy which is accumulated in the fluorescent substance by the aforementioned absorption is radiated as fluorescent light, and the fluorescent light is detected, so that the image can be formed. Concretely, this method is disclosed in, for example, U.S Pat. No. 3,859,527, or Japanese Patent Specification Open to Public Inspection No. 12144/1980. According to these disclosures, a stimulative fluorescent substance is used, and a radiographic image conversion method in which a visible light or infrared ray is used as a stimulation excitation light, as follows. A radiographic image conversion panel which a stimulative fluorescent substance layer is formed on a support, is used; the stimulative fluorescent substance layer is irradiated with the radioactive rays which have passed through a subject, and radioactive ray energy corresponding to radioactive ray transmittance of each portion of the subject is accumulated to form a latent image; then, the stimulative fluorescent substance layer is scanned by the aforementioned stimulation excitation light, and thereby the radioactive ray energy which is accumulated in each portion of the radiographic image conversion panel is radiated and converted into light; and a light signal according to the strength of the light is detected by a photoelectric conversion element such as a photo-multiplier, a photodiode, or the like so that the radiographic image information can be obtained.
On the other hand, there is a method in which: radioactive rays which have transmitted through the subject is absorbed in an uniformly charged semiconductor panel having a photoconductive layer composed of selenium, silicon, or the like so that a latent image can be formed, and after that, the electrostatic latent image on the panel is electrically detected by optical scanning of the semiconductor panel so that the image can be formed. (For example, refer to Japanese patent Specification Open to Public Inspection No. 31219/1979).
The radiographic image information thus obtained, is used without being further processed, or after being image-processed in order to improve the diagnostic property of an X-ray image, by conducting spatial frequency processing or gradation processing, and is outputted onto a silver halide film, or a CRT to be visualized, or stored in an image memory such as a semiconductor memory, a magnetic memory, an optical disc memory or the like, and after that, it is read out as necessary from these image memories and outputted onto a silver halide film, a CRT or the like to be visualized.
As one of the aforementioned spatial frequency processing methods, the following method has been known in which an unsharpness mask signal Sus is obtained, and an original image signal Sorg, the unsharpness mask signal Sus and an emphasis factor .beta. are used in the following equation, EQU S'=Sorg+.beta.(Sorg-Sus)
Due to this equation, spatial frequency emphasis, so-called edge emphasis can be conducted so that the sharpness of the image can be improved. (Refer to Japanese Patent Examined Publication No. 62372/1987.)
However, in the above-described spatial frequency processing method, there is a problem in which a large number of operations are required, and it is difficult to conduct image processing in real time. Further, in the above-described method, there are problems in which image noises are emphasized since a high spatial frequency area is emphasized, and thereby, image quality is lowered due to the image noises. In particular, the method has a disadvantage in that the image quality is extremely low in a portion in which a quantity of radiation is small (the signal is small).
In view of the foregoing, an object of the present invention is to provide a new frequency processing apparatus in which arithmetic processing is simple, and the processing speed is improved. Another object of the present invention is to provide a new frequency processing apparatus in which image quality of a portion having a small signal is not deteriorated due to noises even when contrast (sharpness) of a high frequency area is improved. The first object of the present invention is to provide a new frequency processing apparatus in which image quality deterioration of a portion having a small signal due to noises can be improved while contrast in the high frequency area is maintained.